Research involves study and comparison of normal events during hamster cycle, pseudopregnancy and early post-partum. Special attention is paid to condition of ova at comparable post-ovulation ages during these physiological states. Control of implantation and induced decidualization, life span of the corpora in normal, surgically IUD-bearing or endocrine altered conditions are being studied. Possible immunological aspects in reproduction, and definition of the normal antibody-mediated response in the hamster is under investigation. All studies are being carried out in animals of known age and reproductive history, and the effect of gerontology and parity assessed. Study of ova in pregnant or pseudopregnant animals has been extended to rabbit and to ferret, where effect of IUD is also included.